Many well tools, such as well plugs (bridge plugs, packers, etc.) comprise a sealing device and an anchor device connected to a mandrel. The plug has a radially retracted, or run, state and a radially expanded, or set, state. In the set state, the anchoring device is in contact with the inner surface of the well pipe, and prevents the plugging device from moving axially within the well pipe. In the set state, the sealing device is sealing off the annular space between the mandrel of the plug and the inner surface of the well pipe in order to prevent fluid flow between the lower side of the sealing element and the upper side of the sealing element.
The sealing device comprises a sealing element designed to retract and expand between its run and set states and must also be designed to withstand a high pressure difference and also to be able to seal the annular area at high temperatures. In order to do so, the sealing element, typically made from an elastomer, a rubber material etc., must be supported by supporting devices in the set state.
One such plug is shown in U.S. Pat. No. 7,178,602. Here, the sealing device comprises a sealing element and supporting devices on its upper and lower sides. Each supporting device comprises a number of first supporting arms and a number of second supporting arms having their first ends pivotably connected to a supporting ring provided around the mandrel and where their second ends are pivotably connected to each other. This principle is used in the commercially available High Expansion Retrievable Bridge plug (HEX plug), sold and marketed by Interwell (http://www.interwell.com/hex-retrievable-bridge-plug/category178.html).
One of the HEX plugs is made for use in a 7″ 29 pounds/feet well pipe, where the specification for such pipes allows the inner diameter of the pipe to vary in a range between ca 154.6-159.8 mm, i.e. a variation in the distance between the outside of the supporting arms of the plug in its set state to the inner surface of the well pipe up to 3 mm. The supporting devices can not be made to expand to the largest possible diameters of these pipes, because then, when set in a narrower pipe (i.e. close to 154.6 mm), the supporting devices will contact the pipe surface before a sufficient compression of the sealing element has been achieved.
The HEX is tested and approved for a differential pressure of 4000 psi at a temperature T=110° C. according to ISO 14310:2008 up to validation grade V0.
At higher temperatures and/or higher pressures, the material of the sealing element may be extruded into the gap between the supporting devices and the inner surface of the well pipe in particular if the plug is set in an well pipe having a large inner diameter (i.e. close to 159.9 mm).
One common way to reduce the extrusion of the sealing element is to incorporate a supporting ring within the sealing element itself, such as shown in FIG. 6a-c of WO 2014/016408 and in WO 2012/164051.
One or more embodiments of the present invention may provide a sealing device which can be used to seal well pipes at higher pressures and/or higher temperatures than the above sealing device. In particular, is to one or more embodiments may provide a sealing device which can be used to seal well pipes having a varying inner diameter and to provide a sealing device where the extrusion of the sealing element is reduced and/or prevented in such well pipes.